Cementation product and use for cementing oil wells or the like

ABSTRACT

The present invention provides a cementing composition for an oil well or the like with porosity of less than 50%, and with a solid phase constituted by 35% to 65% (by volume) hollow microspheres, 20% to 45% Class G Portland cement, and 5% to 25% of Class G Portland micro-cement. The invention is used to cement conductor pipes in arctic zones or in deep water holes.

The present invention relates to techniques for drilling oil, gas,water, or geothermal wells or the like. More precisely, the inventionrelates to cementing compositions, more particularly to those adapted tolow temperatures.

After drilling an oil well or the like, a casing or liner is loweredinto the hole and cemented over all or a part of its depth. Cementing isintended to fix the casing and also to prevent fluid being exchangedbetween the various formation layers traversed by the hole, to preventgas from rising via the annular space surrounding the casing, and tolimit the ingress of water into the production well. The cementingoperation consists of injecting a cement slurry via the interior of thecasing, and displacing it by means of another fluid, generally adrilling mud. When it reaches the bottom of the hole, the slurry isconstrained to rise via the annular space between the wall of the holeand the casing. After positioning, the mechanical strength of thehardened cement increases to reach a maximum after about ten days.However, drilling can be resumed as soon as the compressive strengthreaches 3.44 MPa (500 psi); in practice, then, it is desirable to havecompositions with a short Waiting On Cement, i.e., a short period thatelapses between pumping and the time by which the cement has developedsufficient strength to support the conductor pipe.

In the particular case of offshore drilling, special care must be takenwith the first portion of the casing, known as the conductor pipe, whichacts as a guide for subsequent drilling and as a result must beparticularly precisely orientated. The conductor pipe is located a shortdistance beneath the sea bed, at a temperature typically of the order of4° C., while the slurry is prepared on the surface, at a temperaturewhich can be close to the temperature of the well (in the case of aNorth Sea well, for example), but it can also be much higher sincedeepwater wells are often located in tropical or equatorial zones (inparticular the Gulf of Mexico and West Africa). This constitutes a veryparticular case since when cementing the vast majority of oil wells, thecement slurry heats up as it descends in the well.

A cement sets more slowly at lower temperatures. At a few degrees abovezero, an ordinary cement will set only after several days have elapsedduring which period the platform is immobilized and drilling cannot beresumed. Further, the conductor pipe is raised before the cementingoperation and is held temporarily in a winch until the cement issufficiently strong for it act as a support. The longer this stage, themore difficult it is to prevent the conductor pipe from deviating fromits desired orientation.

Various additives aimed at accelerating setting are known, but suchextreme conditions are beyond their capabilities and the quality of thecement slurry and the hardened cement is severely affected. Formulationshave thus been developed which are based on specific cements. They areessentially divided into two classes: formulations based on plaster andformulations based on aluminous cements. Formulations based on plaster,or more exactly a plaster/Portland cement mixture, are generallyintended particularly for logistical purposes; the performance ofaluminous cements is severely affected when contaminated with Portlandcements and they must therefore be stored in separate silos.

In addition, the sea bed is often sandy, with poor consolidation. Thuslow density cement slurries must be used, with a density generally inthe range 11 pounds per gallon (ppg) to 13 ppg, (i.e., 1.32 g/cm³ to1.56 g/cm³). In general, a cement slurry is lightened by increasing thequantity of water and—to avoid the liquid and solid phases separating—byadding compounds such as bentonite or sodium silicate to form gels.While the water/solids weight ratio for an ordinary cement is normallyin the range 38% to 46%, that for a slurry of such low density isroutinely greater than 50%, or even greater than 60%. Such quantities ofwater retard the development of compressive strength and thus prolongthe Waiting On Cement.

A slurry can also be lightened by adding light materials such as silicadust (French patent FR-A-2 463 104) or hollow ceramic or glass beads(United States patents U.S. Pat. No. 3,804,058; U.S. Pat. No. 3,902,911or U.S. Pat. No. 4,252,193). Such materials can reduce but not dispensewith the quantity of additional water added to the cement slurry tolighten it such that the compressive strength development is lessretarded. The quantity of water required remains high and after 24hours, the compressive strength remains very low, generally notexceeding 600 psi (4136 kPa).

A slurry can also be lightened by injecting gas or air. T. Smith, R.Lukay and J. Delorey, in World Oil, May 1984, proposed the use of suchfoamed cements to cement conductor pipes in deep water holes. U.S. Pat.No. 5,484,019 also discloses the use, as a base for such foamed cements,of a slurry comprising 1% to 30% of micro-cement. The principal aim ofU.S. Pat. No. 5,484,019 is to provide a slurry which is capable ofrapidly forming a gel which is sufficiently strong to prevent theingress of water, not to provide a rapid set slurry, in particular fromthe point of view of developing compressive strength.

In any comparison with existing systems, it is important to note thatthe temperature of sea beds depends both on their depth and on theirlocation. Thus while the temperature off the Shetland Isles is no morethan 5° C. from a depth of 500 meters, at the same depth off Malaysia itis about 10° C., and 5° C. is only reached at a depth of 1000 meters.Further, the temperature of the cement slurry depends on the temperatureof the sea bottom and on the existence of submarine currents. In a warmregion of the planet, with few currents, in practice the slurrytemperature can be as much as 15° C. even when the sea bed is at 4° C.In contrast, in a cold region subjected to substantial currents, theslurry temperature can be almost identical to the water temperature. Inthe oil well industry, any operation at a temperature of less than 30°C. is a termed a low temperature application. The present invention isaimed at applications at temperatures which are more particularly in therange 4° C. to 10° C.

The present invention aims to provide novel low density and very lowtemperature cementing formulations which can develop compressivestrength rapidly.

The invention provides cementing compositions with a porosity of lessthan 50% and with a solid phase constituted by 35% to 65% (by volume) ofhollow microspheres, 20% to 45% of Class G Portland cement and 5% to 25%of Class G Portland micro-cement. The term “porosity” means the ratio ofthe volume of liquid in the slurry to the total slurry volume.

The micro-cement used for the compositions of the invention is anessentially pure micro-cement, i.e., constituted by more than 90% ClassG Portland cement. Micro-cements with a maximum particle size in therange 6 μm to 12 μm, preferably 8 μm to 11 μm, are particularlypreferred.

The microspheres used in the invention have low density, preferably lessthan 0.8. Silico-aluminate or cenospheres, a residue obtained from coalcombustion, with an average diameter of the order of 150 μm, areparticularly suitable. Hollow glass beads with an average diameter of120 μm to 250 μm are also suitable.

In general, a dispersing agent is added to the composition as well as acement-setting accelerator. Known dispersing agents generally have aretarding effect on cement setting which must be compensated for. Otherconventional additives can be added, in particular anti-foaming agents,fluid loss control agents or gas migration control agents. Thecomposition of the invention can also comprise a cement-settingaccelerator, in particular calcium chloride, in an amount not exceeding2%, preferably 1.5% (percentage by weight with respect to the weight ofthe solid cement/micro-cement/micro-sphere mixture); adding calciumchloride has a deleterious effect on the rheology of a slurry whichincreases the quantity of dispersing agent which cancels out the effectof the cement-setting accelerator.

Preferably, the solid particles of the mixture are in respectiveproportions such that the compactness of the mixture is close to itsmaximum value. Adding fine particles can thus produce a PVF (PackingVolume Fraction) which is preferably more than 0.75 and more preferablymore than 0.8. In this way, mixing the formulation causes no particularproblems even with porosities as low as in the case of the invention.Further, very satisfactory rheologies are obtained which are favorableto good pumping conditions in particular with an almost complete absenceof sedimentation.

Other advantageous details and characteristics of the invention becomeapparent from the description below of tests carried out on differentexamples of additive compositions.

CHARACTERISTICS OF MICRO-CEMENTS

The majority of oil industry applications using a micro-cement usecompounds formed from slag which comprises 45% lime, 30% silica, 10%alumina, 1% iron oxides, and 5-6% manganese oxide (only the principaloxides are mentioned here; these amounts can, of course, vary slightlydepending on the supplier). This type of micro-cement is termed“micro-slag” below.

Class G Portland cement typically comprises about 65% lime, 22% silica,4% alumina, 4% iron oxides, and less than 1% manganese oxide. Of course,the formulations vary depending on the supplier but the lime/silicaratio is of the order of 3 which is not the case with micro-slag;further, the alumina content of Class G Portland cement is about halfthat of micro-slag. Micro-cement formed from Class G Portland cementwill hereinafter be termed micro-cement G.

The two types of micro-cement tested had very similar granulometriccharacteristics, with a median particle diameter of about 4 μm, alimiting particle size of 12 μm for the micro-slag and 11 μm for themicro-cement G and a specific surface area per unit mass determined bythe air permeability test [Blair Fineness: 0.8000 m²/g].

The two micro-cements were tested at low (10° C.) and very low (4° C.)temperatures. For each slurry tested, it was initially checked that thesystem could be pumped on the surface and injected into the well, acriterion which is considered to be satisfactory when the rheology ofthe slurry, at laboratory temperature and at 10° C., is such that theplastic viscosity of the slurry is less than 250 mPa.s and its yieldpoint is in the range 0 to 9.5 Pa, preferably in the range 0 to 7 Pa.

For these systems deemed to be “pumpable”, the development ofcompressive strength during cement hardening was evaluated by ultrasound(“Ultrasonic Cement Analyzer”), the temperature of the measuring cellbeing controlled using a cooling circuit constituted by a coiled tube inwhich a water/anti-freeze mixture circulated, cooled by a cryostat.Those measurements served to determine the setting time required toobtain a given strength, and also the compressive strength Rt obtainedafter a given time (24 or 48 hours) at a pressure of 3000 psi (20.7MPa).

Further, for these “pumpable” systems, the thickening time TT wasmeasured, which was a measure of the cement pumpability period for thosetests and corresponded to the period required to develop a consistencyof 100 Bc (dimensionless Bearden units); this measure was made, unlessotherwise indicated, at a pressure of 1000 psi (6.9 MPa). In general, asystem was satisfactory if the thickening time was in the range 3 hoursto 6 hours. Tests carried out at other pressures (between 3.4 MPa and13.8 MPa) have shown that the result varied little as a function of thepressure variations.

EXAMPLE 1 MICRO-SLAG IN FRESH WATER

A series of slurries was prepared with a solidcement/cenospheres/micro-slag mixture, in the proportion 35:55:10 byvolume. The slurry porosity was fixed at 42%. The mixing water was amixture of tap water, 2.5 liters of an anti-foaming agent per ton ofsolid cement/cenospheres/micro-cement and a variety of additives shownin the Table below in which the quantities indicated for the dispersingagent, the fluid loss control agent and the sodium silicate(accelerator) are in liters per ton of solidcement/cenospheres/micro-cement mixture. Certain compositions comprisedcalcium chloride as an accelerator (the percentage shown was then apercentage by weight of the solid cement/cenospheres/micro-cementmixture).

The fluid loss control agent used here was an additive which wasparticularly suitable for low temperature cementing, in this case asuspension of a micro-gel obtained by chemically cross-linking apolyvinyl alcohol, by reacting the polyvinyl alcohol in solution withglutaraldehyde at a pH in the range 2 to 3, the molar concentration ofthe cross-linking agent with respect to the monomeric PVA moieties beingin the range about 0.1% to 0.5% in the presence of 3.5% ofpolyvinylpyrrolidone. This additive has been described in detail inFrench patent application FR-A-2 759 364 the contents of which arehereby incorporated by reference.

The dispersing agent was a sulfonated formaldehyde-melamine condensate,a dispersing agent known for its low retarding effect on setting time.

The slurry rheology was measured at laboratory temperature (rheologyafter mixing) or after 10 minutes of conditioning at 10° C.

The Table below shows that for a given slurry, the setting time coulddouble when the temperature was lowered from 10° C. to 4° C. Thedispersing agent had a very large retarding effect at very lowtemperature, which effect was not observed at 10° C.: thus the increasein the quantity of calcium chloride (tests #4 and #5) was without effectat 4° C. because of the increase in the quantity of dispersing agent(for those tests, similar slurry rheologies were sought, although thecalcium chloride had a viscosifying effect which had to be compensatedfor by increasing the dispersing agent).

#1 #2 #3 #4 #5 Density (g/cm³) 1.47 1.47 1.47 1.48 1.48 Dispersing agent(l/t) 6.67 8.35 8.35 11.69 13.35 Fluid loss control agent 50.07 50.0750.07 50.07 50.07 (l/t) Sodium silicate (l/t) 8.35 12.52 16.69 — —Calcium chloride (%) — — — 1.25 2 After mixing: Yield point 1.9 2.5 2.99.6 5.1 (Pa) Plastic viscosity (mPa.s) 122 131 131 116 105 At 10° C.:Yield point 1.0 3.1 9.0 12.2 11.8 (Pa) Plastic viscosity (mPa.s) 208 191193 171 181 TT at 10° C.  8:24  9:39  8:17  6:35  5:55 TT at 25° C. 4:34  3:50  4:26  4:27  2:48 SETTING at 10° C.: time to: 0.35 MPa [50psi] 14:00 17:53 16:38 12:16  9:16 (hr:min) 3.45 MPa [500 psi] 20:0025:37 24:21 17:31 14:51 (hr:min) Rt after 24 hr (MPa) 7.6 6.9 Rt after48 hr (MPa) 12.4 14.1 17.3 18.5 13.1 SETTING at 4° C.: time to: 0.35 MPa[50 psi] 26:58 22:00 28:44 16:56 16:08 (hr:min) 3.45 MPa [500 psi] 39.3328:30 45:04 30:37 24:27 (hr:min) Rt after 48 hr (MPa) 5.9 12.1 4.1 6.210.7

EXAMPLE 2 MICRO-SLAG IN SEA WATER

The solid mixture used in Example 1 was used, with the same porosity butusing sea water as the mixing water.

#6 #7 #8 #9 #10 Density (g/cm³) 1.477 1.477 1.477 1.477 1.474 Dispersingagent (l/t) 19.20 19.20 14.19 14.19 8.34 Fluid loss control agent 25.016.69 16.69 8.34 41.73 (l/t) After mixing Yield point 0.5 −4.9 4 5 25(Pa) Plastic viscosity (mPa.s) 114 148 123 115 181 10° C. Yield point(Pa) 0.5 5 7 29 Plastic viscosity (mPa.s) 205 206 197 284 TT at 10° C.,6.9 MPa >10:0 10:00 11:15 SETTING at 10° C.: 0.35 MPa (hr:min) 16:493.45 MPa (hr:min) 22:08 Rt after 24 hr (MPa) 4

#11 #12 #13 Density (g/cm³) 1.479 1.485 1.489 Dispersing agent (l/t)8.34 8.34 15.0 Fluid loss control agent (l/t) 41.73 41.73 50.07 Calciumchloride (%) 0.5 1 1.25 Rheology after mixing at 25° C. Yield point (Pa)30 31 11 Plastic viscosity (mPa.s) 180 285 140 Rheology at 10° C. Yieldpoint (Pa) 14 Plastic viscosity (mPa.s) 233 TT at 10° C., 6.9 MPa 11:04

With sea water, satisfactory rheology was only obtained by greatlyincreasing the quantities of dispersing agent and the amount of agentrequired was higher as the quantity of fluid loss control agent wasincreased. The retarding effect observed with the slurries prepared withtap water was still further reinforced, such that the thickening timefor some of the “pumpable” slurries was too long and, naturally,accompanied by very slow development of compressive strength, as shownby test #9.

As for the sea water tests, the viscosifying effect supplied by thecalcium chloride was observed again, meaning that the quantity ofdispersing agent had to be increased, practically canceling out theaccelerating effect of the calcium chloride.

EXAMPLE 3 MICRO-CEMENT G IN SEA WATER

The solid mixture used in Example 1 was used, with the same porosity butusing sea water as the mixing water.

#14 #15 #16 #17 #18 Density (g/cm³) 1.48 1.48 1.48 1.48 1.48 Dispersingagent (l/t) 12.52 16.69 19.2 20.86 20.86 Fluid loss control agent 50.0750.07 50.07 25.0 8.35 (l/t) After mixing Yield point 57.4 16.0 3.5 6.87.6 (Pa) Plastic viscosity (mPa.s) 227 167 88 117 112 10° C. Yield point(Pa) 67.8 28.7 4.5 7.3 8.4 Plastic viscosity (mPa.s) 381 275 161 197 174TT at 10° C., 6.9 MPa  5:57 ≈5:00  3:45 SETTING at 10° C.: 0.35 MPa(hr:min) 13:49 n.m 11:43 3.45 MPa (hr:min) 16:58 n.m 14:23 Rt after 24hr (MPa) 12.4 n.m 12.1 Rt after 48 hr (MPa) 25.4 n.m 14.7 SETTING at 4°C.: 0.35 MPa (hr:min) 20:58 n.m 20:32 3.45 MPa (hr:min) 25:46 n.m 25:50Rt after 48 hr (MPa) 17.8 n.m 14.5

Setting at 10° C. and 4° C. was not studied in detail for slurry #17.However, this slurry satisfied the criteria of the invention and thesetting time and compressive strength were estimated to be intermediatebetween the values measured for slurries #16 and #18.

Replacing the micro-slag with class G micro-cement enabled slurries tobe prepared with sea water which developed remarkably high compressivestrengths at low and very low temperatures.

EXAMPLE 4 MICRO-CEMENT G IN FRESH WATER

The two examples below demonstrate that Portland class G cement can alsobe used in fresh water, even in the absence of a specific cement-settingaccelerator.

#19 #20 Density (g/cm³) 1.47 1.47 Dispersing agent (l/t) 14.19 14.19Fluid loss control agent (l/t) 8.35 50.07 After mixing Yield point (Pa)2.7 7.7 Plastic viscosity (mPa.s) 123 123 TT at 10° C., 6.9 MPa  4:35 5:42 SETTING at 10° C.: 0.35 MPa (hr:min) 12:00 14:48 3.45 MPa (hr:min)16:30 19:11 Rt after 24 hr (MPa) 7.72 6.2 Rt after 48 hr (MPa) — 18.7SETTING at 4° C.: 0.35 MPa (hr:min) 25:26 3.45 MPa (hr:min) 31:50 Rtafter 48 hr (MPa) 10.3

EXAMPLE 5

Portland cement is divided into 8 categories, A to H, depending on thedepth, temperature and pressure to which they are exposed. Classes A, Band C are particularly intended for low temperature applications. ClassC cement is considered to be particularly suitable for applicationsnecessitating rapid compressive strength development and thus appears tobe an excellent candidate for very low temperature applications. Class GPortland cement is the most routinely used cement for medium temperatureapplications (typically of the order of 60° C.).

Three cement slurries were prepared with class A, C and G Portlandcement. The rheological properties and the setting characteristics weremeasured.

Cement class A C G Rheology after mixing at 25° C. Yield point (Pa) 20.246.1 9.7 Plastic viscosity (mPa.s) 269 586 184 Thickening time at 25° C. 4:36 2:28  6:10 SETTING at 4° C.: Time to: →0.35 MPa [50 psi] (hr:min)11:00 19:30 →3.45 MPa [500 psi] (hr:min) 19:47 — 24:11 Compressivestrength after 48 hr (MPa) 2077 — 2680

The best rheology was shown by the slurry prepared with the class Gcement. With the class C cement, too viscous a slurry was obtained withtoo high a yield point. With class A cement, the rheology was lesssatisfactory, and on the limits of acceptability but the transition time(from 50 to 500 psi) was close to nine hours Further, the thickeningtime at ambient temperature was only slightly more than 4 hours, whichcould cause problems if the pumping operation was held up for anyparticular reason. It should also be noted that the cement was poorlydispersed and there were problems with free water formation in theslurry. The class G cement had a transition time of less than 5 hoursand produced a better compressive strength after 48 hours.

EXAMPLE 6

For these tests, carried out with fresh water, the micro-cement G usedabove was replaced with other micro-cements based on finer class GPortland micro-cement, with a maximum particle size of 8 μm and 6 μmrespectively.

8μ 6μ Dispersing agent (l/t) 14.19 14.19 Fluid loss control agent (l/t)50.07 50.07 After mixing Yield point (Pa) 1.6 10.3 Plastic viscosity(mPa.s) 126 143 TT at 10° C., 6.9MPa  7:47  6:53 SETTING at 10° C.: 0.35MPa (hr:min) 19:27 13:00 3.45 MPa (hr:min) 24:38 16:11 Rt after 24 hr(MPa) 3.4 15.1 Rt after 48 hr (MPa) 15.4 28.3 SETTING at 4° C.: 0.35 MPa(hr:min) 18:54 24:10 3.45 MPa (hr:min) 25:05 30:13 Rt after 48 hr (MPa)20.0 12.07

A class G portland type micro-cement finer than the micro-cement usedabove could be profitably used, however choosing a very fine cementcaused an increase in the rheology of the slurry over micro-cements witha maximum particle size in the range 7 μm to 12 μm.

What is claimed is:
 1. A well cementing composition, comprising a slurryof: i) a solid phase comprising 35% to 65% by volume hollowmicrospheres, 20% to 45% by volume Class G Portland cement, and 5% to25% by volume Class G Portland micro-cement; and ii) water in an amountsuch that the porosity, being calculated as the ratio of the volume ofliquid to the total volume of slurry, is less than 50%.
 2. A cementingcomposition according to claim 1, characterized in that the maximumparticle size of the class G Portland type micro-cement is in the range6 μm to 12 μm.
 3. A cementing composition according to claim 1,characterized in that the mixing water is sea water.
 4. Cementingcomposition according to claim 2, characterized in that the mixing wateris sea water.
 5. A cementing composition according to claim 1,characterized in that it also contains a dispersing agent.
 6. Acementing composition according to claim 2, characterized in that italso contains a dispersing agent.
 7. A cementing composition accordingto claim 3, characterized in that it also contains a dispersing agent.8. A cementing composition according to claim 1, characterized in thatit also contains a fluid loss control agent.
 9. A cementing compositionaccording to claim 2, characterized in that it also contains a fluidloss control agent.
 10. A cementing composition according to claim 3,characterized in that it also contains a fluid loss control agent.
 11. Acementing composition according to claim 5, characterized in that italso contains a fluid loss control agent.
 12. A cementing compositionaccording to claim 1, characterized in that it also contains acement-setting accelerator.
 13. A cementing composition according toclaim 2, characterized in that it also contains a cement-settingaccelerator.
 14. A cementing composition according to claim 3,characterized in that it also contains a cement-setting accelerator. 15.A cementing composition according to claim 5, characterized in that italso contains a cement-setting accelerator.
 16. A cementing compositionaccording to claim 8, characterized in that it also contains acement-setting accelerator.